The present invention relates to an apparatus for measuring the mass flow rate of a powderous body flowing in a pipe arrangement using an electrostatic capacity detector and a mutual correlation method.
In a flow meter of this type, it is difficult to maintain desirable precision in the measured value of the flow rate since the flowing manner of a powdered body is in general widely changed in accordance with the grain size and the specific gravity of the body, the pressure and the velocity of flow of the carrier gas, and the formation of the pipe arrangement.
Referring to FIG. 1, the arrangement of a conventional flow meter of this type is shown. In FIG. 1, a powder 2 flows in a pipe 1 in the direction shown by the arrow. An electrode plate 3 having a large width is mounted on the outer wall of the pipe 1 to detect the density of the powder body, and electrode plates 4 and 5 having a narrow width are mounted on the outer wall of the pipe 1 to detect the velocity of flow of the powder body. The electrode plate 4 is positioned apart from the electrode plate 5 by the distance L. The electrostatic capacity is increased when the powder body 2 arrives at the density detecting electrode 3, and the increment thereof is converted to an electric signal by an electrostatic capacity converter 6 to represent a density indication .rho..
On the other hand, the change in electrostatic capacity due to the velocity detecting electrodes 4 and 5 is converted to an electric signal u.sub.1 at upstream side and an electric signal u.sub.2 at downstream side by electrostatic capacity converters 7 and 8, respectively. A velocity meter of the correlation type receiving the upstream signal u.sub.1 and the downstream signal u.sub.2 effects mutual correlation between the upstream and downstream signals u.sub.1 and u.sub.2, and provides a delay time .tau. therebetween, and further operates to calculate v=L/.tau. to produce an output signal representing the flow velocity v. The output of the mass flow meter is obtained by multiplication of the density indication .rho. and the flow velocity indication v using a multiplexer 10.
The above-mentioned system for obtaining the mass flow rate of a powder body is known from publications such as "Iron and Steel", April 1968, page 5. Such a system has the following disadvantages. One disadvantage relates to the electrode plates 4 and 5 for detecting the flow velocity. The electric force lines produced between the velocity detecting electrode plates 4 and 5 mounted on the wall of the pipe 1 are formed as shown in FIG. 2. Therefore, the electrode plates detect the powder body over a wide range. Especially in the case where the upstream side electrode plate 4 is located adjacent to the downstream side electrode plate 5, the electric force lines produced by the upsteam and downstream side electric plates 4 and 5 overlap each other. In this case, the separation between the output signals obtained by the electrode plates 4 and 5 becomes unclear to thereby round off the peak point of the correlation curve obtained.
In the case where ultrasonic oscillating elements 11 and 13 and ultrasonic receiving elements 12 and 14 are used for detection as shown in FIG. 3, or in the case where a light source and a light receiving element are used for detection, the possible range of detection is narrow and further the boundary becomes clear. However, detecting methods such as those just mentioned are not suitable for the measurement of the flow rate of a powder body, because absorption and dispersion due to the powder body are large, so that the ultrasonic waves and light cannot reach the receiver elements 12 and 14.
The other of the disadvantages of the abovementioned system relates to the density detection. In a density meter of this type, the amount of change in electrostatic capacity, which represents the density of the objective component to be measured, is very small in comparison to the value of the electrostatic capacity between the vessel and medium. Therefore, since the absolute value of the electrostatic capacity of the objective component is very small, large errors due to temperature change are undesirably produced even if the electrostatic capacity between the vessel and the medium is only slightly changed in accordance with the change in ambient temperature and that of the medium. For example, in the case shown in FIG. 4, where electrode plates 42 and 43 having a length dimension of 15 cm and a width of 3 cm, respectively, are mounted on an outer wall of a ceramic pipe 41 having an external diameter of 35 mm and an internal diameter of 25 mm, the detected value of the electrostatic capacity between the electrode plates 42 and 43 is about 20 pF with air in the ceramic pipe 41, and is increased by about 0.2 pF when the powder body is located within the ceramic pipe 41. At that time, if the change in the electrostatic capacity of the ceramic pipe with respect to temperature is 100 ppm/.degree. C., a change of 0.02 pF may be produced by a temperature change 10.degree. C. This corresponds to an error of 10% when the density of the powder body is maximally set at 0.2 pF.